In the semiconductor industry, various processes can be used to deposit or remove materials on or from the surface of wafers. For example, electrochemical deposition (ECD) or electrochemical mechanical deposition (ECMD) processes can be used to deposit conductors, such as copper, on previously patterned wafer surfaces to fabricate device interconnect structures. Once the conductor is deposited on the wafer surface to fill various features such as trenches and vias, excess conductor, which is also called overburden layer, often needs to be removed. Chemical mechanical polishing (CMP) is commonly used for this material removal step. Another technique, electropolishing or electroetching, can also be used to remove excess materials from the surface of the wafers. Electrochemical (or electrochemical mechanical) deposition of materials on wafer surfaces or electrochemical (or electrochemical mechanical) removal of materials from the wafer surfaces are collectively called “electrochemical processing”. Electrochemical processing techniques include, but are not limited to, electropolishing (or electroetching), electrochemical mechanical polishing (or electrochemical mechanical etching), electrochemical deposition and electrochemical mechanical deposition. All the above techniques utilize a process solution.
As generally exemplified in FIG. 1, an ECD system 10 contains a chamber 12 including an electrode 14. The electrode is used as an anode for the deposition processes. However, the electrode may also be polarized as a cathode, if an electroetching or electropolishing process is employed. A carrier head 16 having a rotatable shaft 18 holds a wafer 20 in a process solution 22, which is delivered to the chamber 12 through a solution inlet 24. The solution leaves the chamber 12 from an upper end 26 of the chamber in the direction of arrow A for recycling, re-furbishing or discarding. For example, for copper deposition, the wafer is usually a preprocessed wafer having features or cavities on the surface, which are typically coated with conductive layers such as barriers and seed layers. During electrochemical processing the wafer is lowered into the process solution 22 and preferably rotated while a potential difference is applied between the wafer 20 and the electrode 14. The potential difference is applied by a power supply, which is connected to the electrode and the conductive wafer surface using suitable electrical contacts (not shown).
One difficulty in such a process is that as the wafer is lowered into the process solution, gas bubbles 28 may be trapped under the wafer 20. If the process is a deposition process for copper interconnect fabrication, for example, such bubbles prevent copper from depositing onto the bubble-containing regions on the wafer surface, giving rise to un-plated or under-plated areas, which represent defects in the plated material. Such defects reduce the reliability of the interconnect structures. Similarly, in an electropolishing process, trapped bubbles retard material removal from the regions containing the bubbles, giving rise to non-uniformities and defects and cause reliability problems.
In the prior art, various techniques are used to eliminate bubbles trapped under the wafers during entry into process solutions. One such known method requires tilting the carrier head 16 as it enters the process solution to let the bubbles escape. However this approach requires expensive carrier head designs, which increase manufacturing cost.
Therefore, to this end, there is a need for alternative bubble elimination designs and processes, which can be employed during electrochemical processing of a workpiece such as a wafer.